1. Field of the Invention
This invention relates to digital transmission systems in the subscriber loop plant and, more particularly, to detection of the current status of subscribers in a digital transmission system.
2. Description of the Prior Art
In a digital carrier transmission system, such as the T-1 carrier system, the channel units perform two important functions. First, signals on the lines are sampled periodically and pulse amplitude modulated (PAM) samples are sequentially transmitted from the channel units to a transmit-receive unit. Second, signaling information, e.g., requests for service, ringing, and class of service information, are transmitted to the transmit-receive unit.
At the transmit-receive unit, the PAM samples are encoded and multiplexed for transmission as a single bit stream at the DS-1 rate of 1.544 MHz. In every sixth frame, signaling bits are inserted in the least significant bit position of the corresponding code word for all twenty-four multiplexed code words. Because some channel banks can transmit digital data, the class of service information is used to inform the transmit-receive unit when encoding is not necessary.
In a digital concentrator system such as the one disclosed in applicant's copending application (case 2), Ser. No. 186,617, filed of even date herewith, the code words from a plurality of bit streams obtained from two transmit-receive units are demultiplexed and stored alternately in two different sections of a memory device in succeeding frames. To extract the signaling information every sixth frame would require additional real time processing which would considerably slow down and complicate the circuitry. Furthermore, in order to assign a concentrator trunk, it is sufficient for the concentrator system to know when service is requested and when service is terminated, i.e., off-hook or ringing signals and on-hook signals, also known as activity information for the status of a line.